It is known that, for example, automatic flexible manufacturing systems and automatic storage systems involve the feeding of materials, sheets and work pieces for machining, storing or other handling. The work pieces are typically placed on various bases, trays or pallets. Material sheets of various sizes can also be placed in various cassettes which are used as transport supports and in which the materials can also be stored.
Typically, the system also comprises various automatic lift and transfer devices which transfer work pieces in the system, to be processed further or to be stored in a storage rack, and back. Lift and transfer devices include, for example, stacker cranes that handle cassettes, bases, trays, pallets or work pieces and comprise suitable devices, such as transfer forks, telescopic forks, lifting mechanisms, grippers, or the like, particularly for moving bases and cassettes to the stacker crane and back to the rack. Other lift and transfer devices, which are equipped with the above-mentioned devices, if necessary, include transfer devices operating at vertical storages and possibly moving in the vertical direction only, or transfer devices which move in the horizontal direction only and supply cassettes from loading stations or from feeding points of the system into other locations of the system, between locations in a manufacturing system, or into a storage. In a stacker crane, for example, the devices are normally placed in a lifting carriage which moves in the vertical frame structure of the stacker crane, driven by, for example, a cable drive or a chain drive. The lift and transfer device is normally arranged onto the floor level, in a driving carriage which typically moves on the support of rails, normally between long sets of racks. In other lift or transfer devices, these devices are placed, for example, on top of a carriage or sledge moving in various ways.
The lift or transfer devices also transfer pieces to various manufacturing stations with automatic manipulating devices of their own, especially for manipulating a pallet. The lift or transfer device systems are automatically controlled in ways known as such by means of a control program stored in control means, into which control program is entered the information needed, for example, about the pieces to be manipulated, the storage locations and the desired transfers or transfer locations.
When e.g. metal plates are stored in cassettes, the weights of the cassettes increase, and particular attention must be paid to the way in which the cassette is moved from the supports of the rack to the transfer device, for example a lifting carriage. According to FR 2705955, pieces to be collected are placed on bases which are pulled by means of a gripper from a rack into a stacker crane, both being equipped with rolls on which the base moves. However, the problem is that the stacker crane and the base are not precisely at the same height, wherein the box hits the rolls of the stacker crane when the stacker crane is higher than the rack. During and at the end of the transfer, the cassette also moves in a swinging manner on the rolls. Further, document DE 4339056 A1 presents the transfer of bases into a stacker crane, which also requires precise mutual positioning to prevent collisions or strong swingings of the base. A gripper is presented in FR 2138491, in which the base is equipped with rolls and the rack is provided, at corresponding locations, with brackets, along which the base rolls. Also in this case, the transfer of the wheels from the bracket onto the transfer device is critical, and problems are caused by the collision of the wheels with the brackets of the transfer device or by their falling between the bracket of the rack and the transfer device. If the base is equipped with wheels, problems include particularly the collision of the foremost wheel with the structures of the transfer device as well as the collision of its front edge with the idlers of the transfer device. Another problem is the return of the rearmost wheel of the support onto the brackets of the rack.
The collision of the box with the structures of the stacker crane can be easily perceived from EP 0116126 A1 and it may be acceptable in the case of boxes with a light weight, but as the weight of the bases and the materials increase, the force effects caused by collisions are great, causing noise and damaging the structures, thereby increasing the need for repair and maintenance. Heavy structures to be particularly mentioned include cassettes intended for the manipulation and storage of steel plates. Also document EP 0379688 A1 requires precise positioning in the correct location, which may be even impossible if the cassettes are heavy, because the structures may yield, move or be displaced as well, due to the heavy loads.